Answer:
The knight draws out his sword to kill the snake and the fight starts.
Step-by-step explanation:
To understand the passage fully, we should return to what has happened before - Sir Mordred and Arthur met up, but both told their parties that if someone from the other party is seen drawing up a sword, that means they are trying to cheat.
In this passage, we see that, when they met, the snake showed up, stung a knight and he drew out a sword to kill it. But when other people saw this, they assumed it's a start of fraud and called for a battle, so the battle cry started with the trumpets and horns.
In the end, both Sir Mordred and King Arthur returned to their parties, sad for a fact that they have to fight.